This invention relates to a method of manufacturing a chromium oxide film adapted for use as, for example, a photomask.
Where an electronic circuit is integrated with high precision, a photomask is used selectively to project light on a photoresist deposited on a properly worked surface of a substrate. The photowork is prepared by applying, for example, photoetching to a photomask blank consisting of a light-permeable substrate made of, for example, glass, one side of which is coated with an opaque film. A chromium oxide film has been known to be superior in mechanical strength and chemical resistivites as compared with iron oxide film and used as an opaque material for a photomask.
A known process of manufacturing a chromium oxide film comprises the step of evaporating Cr.sub.2 O.sub.3 evaporation source or Cr.sub.2 O.sub.3 target on a substrate in vacuum or by sputtering or ion plating. With this process, however, a source material of evaporation originally consists of a particular form of chormium oxide (Cr.sub.2 O.sub.3) and does not admit of any further control of the degree of oxidation of this compound to a suitable level for providing, in high reproducibility, an opaque film having desired optical properties and mechanical strength. An opaque film produced according to the prior art is also subject to various defects such as projects or pinholes. Another known process of manufacturing a chromium oxide film which has been proposed to eliminate the above-mentioned drawbacks comprises the steps of providing a source material of evaporation consisting of chromium metal alone; evaporating the chromium metal in an atmosphere containing O.sub.2 gas; and controlling the composition of chromium oxide according to the manner in which the chromium metal is reacted with O.sub.2 gas. Where said another known process is carried out by vacuum evaporation, then a filament used as a heat source is prominently deteriorated due to oxidation by said O.sub.2 gas and undesirably decreases in effective life. This event also occurs in the plasma sputtering process. Further where O.sub.2 gas has a high concentration in carrying out other forms of sputtering such as diode sputtering, radio frequency sputtering and ion plating, then difficulties arise in attaining a stable electric discharge or ion bombardment. Still another known chemical process of depositing chromium oxide in the gaseous phase (chemical vapor deposition) requires a substrate to be heated to 500.degree. to 600.degree. C, and is little adpated for practical application due to the substrate being deformed by such high temperature.